A conventional cushion 10 for a chair in accordance with the prior art shown in FIGS. 11 and 12 comprises a cushion body 11 and two fasteners 12 mounted on a rear face of the cushion body 11. Thus, when the cushion is placed on a seat, the cushion body 11 can support a user's rear body to provide a comfortable sensation to the user. However, the cushion body 11 can support the user's waist only and cannot support the user's back so that the user's back is suspended in the air. Thus, the user's back easily feels uncomfortable and is easily deformed during a long-term utilization.